1. Field of the Invention
The invention relates to the technical sphere of geological storage of greenhouse gas such as carbon dioxide (CO2), and more particularly to the monitoring of geological storage sites for such gases.
2. Description of the Prior Art
The scenarios established by the IPCC (Intergovernmental Panel on Climate Change) show that the CO2 concentration in the atmosphere, in the absence of any corrective measure, will evolve from a current concentration of 360 ppm to more than 1000 ppm by the end of the XXIst century with significant consequences on the climate change. Capture of the CO2 emissions from high volume sources (for example, thermal power plants), transportation of this CO2 and storage thereof in suitable underground formations is one of the solutions available for reducing greenhouse gas emissions. CO2 geological storage pilot projects exist already, but continuing the deployment of this technology requires high-quality technologies in order to meet the requirements of the regulations that are being set up and to meet public expectations.
Deep saline aquifers have the highest potential for CO2 geological storage among all the geological formations being considered regarding their geographical distribution and their theoretical storage capacity.
The volume of the CO2 injected in an underground geological formation is easily known by measuring the gas flow rate at the wellhead. However, the fate of the CO2 once injected is much more difficult to control: since CO2 can migrate vertically out of the storage formation (to more superficial geological layers or even to the surface) or laterally into the host formation in non-initially planned zones. Furthermore, the CO2 can undergo physico-chemical changes over time, likely to cause it to take different forms, among which are free form (gaseous or supercritical), dissolved form, in brine, or a mineralized form for example.
Thus, monitoring as completely as possible the fate of CO2 has to be carried out in order to meet the regulations in force, and to help towards societal acceptance of this technology. This complete monitoring must involve detecting leakage out of the geological storage formation and quantifying such leaks, as well as a volume and/or mass balance of the CO2 in place in the geological storage formation.
The following documents reflect the state of the art:    Arts, R. et al., 2002. Estimation of the Mass of Injected CO2 at Sleipner Using Time Lapse Seismic-Data, Expanded Abstracts of the 64th EAGE, Florence 2002, paper H016.    Calvert, R., 2005, Insights and Methods for. 4D Reservoir Monitoring and Characterization, SEG/EAGE DISC (Distinguished Lecture Course), 2005.    Chadwick, R. A., Arts, R. et O. Eiken, 2005, 4D Siesmic Quantification of a Growing CO2 Plume at Sleipner, North Sea, In: DORE', A. G. & VINING, B. A. (eds) Petroleum Geology: North-West Europe and Global Perspectives—Proceedings of the 6th Petroleum Geology Conference, 1385-1399.    Bourbié, T., Coussy, O. et B. Zinszner, 1987, Acoustics of Porous Media, Editions Technip, Paris.    Rasolofosaon P., Zinszner B., 2004, Laboratory Petroacoustics for Seismic Monitoring Feasibility Study. The Leading Edge, v. 23; no. 3, p. 252-258.    Rasolofosaon, P. N. J. and Zinszner, B. E., 2007. The Unreasonable Success of Gassmann's Theory . . . . Revisited, Journal of seismic Exploration, Volume 16, Number 2-4, 281-301.    Zinszner, B. et F. M. Pellerin, 2007, A Geoscientist's Guide to Petrophysics, Editions Technip, Paris.
Many techniques have been developed by industrialists in order to monitor the evolution of the fluids produced or injected in a porous medium. Among these techniques, the repetitive seismic technology, referred to as 4D seismic technology, is used in the (petroleum or environmental) industry. Such a technique carries out various seismic surveys, at different times (the surveys are generally carried out at one year intervals at least). Thus, specialists can follow the evolution of the fluids of the reservoir under production or of the geological storage site (Calvert, 2005, for example).
The seismic data (velocities), which are estimated from the acquired data, allow obtaining the elastic properties of the fluids produced or injected by means of a theoretical model, generally of poroelastic type (Biot-Gassmann) (for example, Bourbié et al., 1987, Rasolofosaon and Zinszner, 2004 and 2007).
All these techniques have been exploited in the environmental sphere to estimate, from seismic data, the total volume and the total mass of CO2 in place in the subsoil.
For example, Arts et al. (2002) exploit the measurements of the delay taken by the seismic wave to travel through the subsoil layers invaded by the CO2, in relation to a faster propagation in the brine-saturated subsoil, in order to locate the CO2-invaded zone and to estimate the total volume of CO2 in place. These authors use Gassmann's theoretical model (for example, Rasolofosaon and Zinszner, 2004 and 2007). Furthermore, assuming that the mean density of the CO2 under the reservoir conditions (pressure and temperature) are known, they can estimate the total mass of the CO2 in place, which they compare more or less successfully with the mass of CO2 really injected.
In a similar but more sophisticated approach, Chadwick et al. (2005) exploit not only the wave propagation time data, but also the amplitudes of the seismic waves. They obtain somewhat finer estimations of the CO2 distribution and of the total mass of the CO2 in place than the previous authors, without however reaching the three-dimensional distribution of the CO2 saturations as provided by the invention.
The major drawbacks of the previous methods can be summed up in two main points:
First, the previous analyses are essentially based on the analysis of the wave propagation times and amplitudes, and not on a complete inversion of the seismic data, with a really quantitative estimation, at any point of the subsoil, of the elastic parameters (impedances, incompressibility moduli, etc.). Now, by analyzing the times or the amplitudes, it is not possible to perform a true quantitative analysis. In fact, if for example the analysis of the propagation time variations in the storage layer due to CO2 injection is performed, an estimation of the overall velocity variation in the entire layer (and not local, at each point) is obtained,
Second, all these methods are based on the use of an elastic model of the porous medium, whose robustness needs no further proof, but whose key parameter estimation (drained incompressibility and shear moduli notably, and grain compressibilities to a lesser extent) still is a problem (for example, Arts 2002 and Calvert et al. 2005).